The present invention relates to a gate driving circuit and gate driving method of a power MOSFET.
To drive the gate of a power MOSFET in, e.g., the power supply of a CPU, it is essential to increase the switching frequency.
FIG. 18 shows an example of a gate driving circuit of a conventional power MOSFET. A MOSFET M1 in the output stage has a drain connected to a power supply voltage VCC terminal, a source connected to a ground voltage VSS terminal, and a gate connected to a resistor Rg which prevents an excess gate current.
A constant input voltage Vin is applied between input terminals 101 and 102. Switching elements SW1 and SW2 are connected in series between the input terminals 101 and 102. A capacitance element C and series-connected diodes D1 and D2 are connected in parallel with the switching elements SW1 and SW2. The connecting point of the switching elements SW1 and SW2 and the connecting point of the diodes D1 and D2 are connected to the gate of the MOSFET M1 via the resistor Rg.
References disclosing conventional gate driving circuits are as follows.
Reference 1: A resonant pulse gate drive for high frequency applications Wiegmen, H. L. N.; Applied Power Electronics Conference and Exposition, 1992. APEC '92. Conference Proceedings 1992., Seventh Annual, 23-27 Feb. 1992 Page(s): 738-743
Reference 2: A resonant MOSFET gate driver with complete energy recovery Yuhui Chen; Lee, F. C.; Amoroso, L.; Ho-Pu Wu; Power Electronics and Motion Control Conference, 2000. Proceedings. PIEMC 2000. The Third International, Volume: 1, 2000 Page(s): 402-406 vol. 1
Reference 3: A novel resonant gate driver for high frequency synchronous buck converter Yao, K.; Lee, F. C.; Applied Power Electronics Conference and Exposition, 2001. APEC 2001. Sixteenth Annual IEEE, Volume: 1, 2001 Page(s): 280-286 vol. 1
Reference 4: A novel resonant gate driver for high frequency synchronous buck converters Kaiwei Yao; Lee, F. C.; Power Electronics, IEEE Transactions on, Volume: 17 Issue: 2, March 2002 Page(s): 180-186
Reference 5: A MOS gate driver with resonant transitions Maksimovic, D.; Power Electronics Specialists Conference, 1991. PESC '91. Record., 22nd Annual IEEE, 24-27 Jun. 1991 Page(s): 527-532
Reference 6: Design of a high speed power MOSFET driver and its use in a half-bridge converter Leedham, R. J.; McMahon, R. A.; Power Electronics and Applications, 1993., Fifth European Conference on, 13-16 Sep. 1993. Page(s): 407-412 vol. 2
Reference 7: Japanese Patent Laid-Open No. 10-52061
Reference 8: Japanese Patent Laid-Open No. 5-207731
Reference 9: Japanese Patent Laid-Open No. 11-308084
FIG. 19 shows losses with respect to the input power in the conventional gate driving circuit as shown in FIG. 18. The losses include a loss caused by gate driving and a conduction loss. The gate driving loss depends on the frequency.
In the conventional gate driving circuit as shown in FIG. 19, the gate driving loss increases as the frequency rises. When the loss increases, the heat generation amount also increases, so a countermeasure for heat radiation is necessary. As a consequence, the size of the whole apparatus increases.
Accordingly, it is conventionally impossible to reduce the driving loss when the frequency is raised.
The gate of a MOSFET has a capacitor structure. In principle, therefore, the gate structure of a MOSFET does not singly generate any loss. A loss is produced by the gate resistor Rg in the gate driving circuit and the parasitic resistance in the circuit. The value of the loss is f×Cg×Vg2 where Cg is the gate capacitance.
If, therefore, the energy consumed by the gate resistor Rg and the wiring resistance can be regenerated, the loss can be effectively prevented at high frequencies. However, this cannot be well done in the conventional circuit.
Also, the maximum value of the gate current of the MOSFET M1 is determined by gate voltage Vg/gate resistor Rg, so it cannot be changed to any desired value in accordance with the situation. Since the gate current gradually increases, the gate voltage transition time, i.e., a so-called Miller time prolongs. This increases the switching loss of the MOSFET to be driven.
Furthermore, in the conventional circuit, an electric current is supplied to the gate of the MOSFET M1 via the resistor Rg, so the gate voltage is not fixed at a low impedance. Consequently, the gate voltage varies when the high-side switching element SW1 and low-side switching element SW2 are alternately turned on, so the circuit may cause faulty operations. In relation to this drawback, the circuit is readily influenced by EMI.